Investigation of artifacts from chronographic tethering experiments—interactions between tethers and predators

We used a submersible chronographic tethering device to compare the survival time of tethered juvenile brown tiger prawns (Penaeus esculentus [Haswell, 1897]) in seagrass and bare plots at two different sites. At one site, more prawns survived and those that were eaten survived for longer among the seagrass than on the bare habitat. However, at the other site there was little difference between numbers of survivors and survival time in the seagrass compared to the bare substrate. Laboratory observations suggest that the difference in survival was due to an effect of the tethers on the prawn's ability to escape from blue-swimmer crabs (Portunus pelagicus [Linnaeus 1758]), and differences in the abundance of P. pelagicus at the two field sites. P. pelagicus rely on olfaction, rather than sight to locate their prey, and so the crabs' efficiency at locating tethered prawns is not affected by the structure that seagrass provides. Our results clearly demonstrate an example of an artifact caused by experimental intervention, and highlight the need to investigate the possible effects of these types of artifacts.